The present invention relates to lighting applications, more specifically to light emitting diode (LED) and laser diode (LD) applications. More specifically, the present invention relates to a homoepitaxial LED or LD structure and a method for making such a structure.
During the past decade there has been tremendous interest in gallium nitride (GaN) based optoelectronic devices, including light emitting diodes and laser diodes. Because high-quality GaN substrates have not been available, virtually all of the art has involved heteroepitaxial deposition of GaN and GaInAlN on sapphire or SiC substrates. A thin low-temperature buffer layer, typically AlN or GaN, is used in order to accommodate the lattice mismatch between GaN and the substrate and maintain an epitaxial relationship to the substrate.
The use of sapphire substrates with a low-temperature buffer layer has a number of important limitations for manufacture of LEDs. Sapphire is an electrical insulator, forcing electrical contacts to be made above and to the side of the device structure, rather than above and below (a so-called vertical device structure), wasting space on the wafer. In addition, sapphire has a rather poor thermal conductivity, limiting heat dissipation. Sapphire has a large (16%) lattice mismatch with respect to GaN, so that even with the use of buffer layers a very high level of threading dislocations (107–1011 cm−2) are generated within the device structure. These dislocations can act as non-radiative recombination centers and may limit performance in certain applications, such as, for example, reducing emission efficiency in near-ultraviolet and high power LEDs and LDs and reducing the lifetime in LDs. Deposition of the low-temperature buffer layer also adds cost and complexity to the process. Sapphire also has a large (45%) mismatch in the thermal expansion coefficient with respect to GaN, which generates stresses in device structures upon cooldown from the processing temperature and limits the maximum size of wafers and thickness of epitaxial layers that can be used without forming cracks. Facets must be prepared at the ends of laser diode structures in order to define the laser cavity, and the difficulty in cleaving c-axis-oriented sapphire makes facet preparation more expensive.
The use of SiC substrates alleviates some of these limitations but introduces other problems. The SiC lattice mismatch to GaN is smaller than that of sapphire, but very high defect concentrations are still generated, and the use of low-temperature buffer films is still needed. SiC is also much more expensive than sapphire. Lower cost SiC is typically opaque, decreasing the efficiency of the LED device because light emitted from the active region toward the substrate would be absorbed rather than transmitted. Since some applications of LEDs involve emission of ultraviolet light; this light could be absorbed by even a high-quality, transparent SiC substrate because the bandgap is less than that of sapphire or GaN.
A high-quality GaN substrate would reduce these problems. The substrate could be made electrically conductive as well as semi-insulating, so vertical LED or LD structures could be fabricated. The thermal conductivity of pure GaN is five times that of sapphire, improving heat dissipation, enabling higher power levels, and improving lifetime. Also, there would be no thermal expansion mismatch, resulting in ease of scalability to larger substrates, which would reduce cost. The concentration of threading dislocations would be reduced by 3–10 orders of magnitude, which would reduce leakage currents, improve device yields, and the consistency of I–V characteristics, increase device lifetimes, particularly at high power levels, and may also improve emission efficiency and resistance to static discharge. Furthermore, GaN is much easier to cleave than sapphire, and LD facets can be prepared by simple cleavage rather than by reactive ion etching, further reducing costs.
Some limited work has previously been carried out on forming homoepitaxial LED or LD devices on GaN substrates. Writing in the Journal of Crystal Growth, Pelzmann, et al. reported that homoepitaxial homojunction GaN LED devices demonstrated a doubling of the emission intensity relative to the analogous device on a sapphire substrate. However, homojunction GaN LEDs have much lower emission intensities than InGaN/GaN heterojunction LEDs, as is well known in the art. Therefore, the devices demonstrated by Pelzmann, et al. do not offer any performance advantages relative to conventional heteroepitaxial LEDs.
Kamp, et al. have developed a method for the formation of GaN crystals with homoepitaxial GaN growth thereon. This work focuses on the application of chemically assisted ion beam etching as a method for polishing the GaN crystal prior to LED formation. Grzegory et al. and Prystawko et al. have reported the fabrication of a pulsed-operation blue LD on a bulk GaN substrate. The homoepitaxial GaN-based LEDs described by Kamp et al. and the LD described by Grzegory et al. and by Prystawko et al. suffer from a number of important limitations that are overcome by the present invention. The single-crystal GaN substrates were grown in molten Ga with a N2 overpressure of 10–20 kbar at a temperature below 1600° C. The undoped GaN crystals grown by this method have a high concentration (about 5×1019 cm−3) of n-type defects, which are believed to comprise oxygen impurities and nitrogen vacancies. As a consequence, the crystals are relatively opaque, with an absorption coefficient of about 200 cm−1 at wavelengths between 700 nm (red) and 465 nm (blue) in the visible portion of the spectrum, and up to half the light emitted by the LED is absorbed by the substrate. This constitutes a large disadvantage compared to conventional heteroepitaxial LEDs fabricated on sapphire or transparent SiC substrates. In addition, the substrates employed by Kamp et al. have a dislocation density of approximately 103 to 105 cm−2. This value is lower than the corresponding values in heteroepitaxial LEDs of approximately 107 to 1010 cm−2 but will still result in dislocations being present in large-area devices. Further, the high concentration of n-type defects in undoped crystals grown in molten Ga causes the lattice constant to increase by about 0.01–0.02%, which generates strain in undoped epitaxial GaN layers deposited thereupon. Additionally, the undoped GaN substrates employed by Kamp et al. have a rather limited carrier mobility, about 30–90 cm2/V-s, which may be limiting in high-power devices.
Porowski, et. al., writing in Acta Physica Polonica A, disclosed a method for growing transparent GaN crystals involving the addition of 0.1–0.5% magnesium to a gallium growth medium at temperatures of 1400–1700° C. and nitrogen pressures of 10–20 kbar. This method is capable of producing GaN crystals with an optical absorption coefficient below 100 cm−1. However, these crystals are semi-insulating, with an electrical resistivity of 104–106 Ω-cm at room temperature, rendering them unsuitable as substrates for vertical light-emitting structures of one type described in the present invention. These substrates have several additional disadvantages, including: (i) a high concentration (approximately 1019 cm−1) of Mg and O atoms, each [J. I. Pankove et al., Appl. Phys. Lett. 74, 416 (1999)], which could potentially diffuse into device structures during high temperature processing; and (ii) relatively poor thermal conductivity.
A final but very important limitation of the method of Porowski et al. is that it does not appear to be scalable; that is, the method is incapable of producing GaN boules and wafers having diameters greater than or equal to 50 mm. Instead, the process typically yields a number of platelet crystals, each having a diameter of about 10 mm and a thickness of 0.1–0.2 mm, with the largest crystal grown to date by this method being about 20 mm in diameter. Because the process of Porowski et al. yields platelets rather than thick boules, the economies of scale associated with conventional wafering technology (slicing, polishing) cannot be achieved.
The most mature technology for growth of pseudo-bulk or bulk GaN is hydride/halide vapor phase epitaxy, also known as HVPE. In the most-widely applied approach, HCl reacts with liquid Ga to form vapor-phase GaCl, which is transported to a substrate where it reacts with injected NH3 to form GaN. Typically the deposition is performed on a non-GaN substrate such as sapphire, silicon, gallium arsenide, or LiGaO2. The dislocation density in HVPE-grown films is initially quite high, on the order of 1010 cm−2 as is typical for heteroepitaxy of GaN, but drops to a value of about 107 cm−2 after a thickness of 100–300 μm of GaN has been grown. For example, Vaudo et al. [U.S. Pat. No. 6,596,079] teach a method of fabricating GaN wafers or boules with a dislocation density below 107 cm−2. Yasan and co-workers, writing in Applied Physics Letters, disclosed a homoepitaxial light-emitting diode fabricated on a free-standing HVPE GaN substrate.
HVPE may be capable of reducing defect levels further in thicker films, but values below 104 cm−2 over an entire wafer appear to be unlikely. Edge dislocations, which normally comprise a significant fraction of the threading dislocations present in heteroepitaxially-grown GaN, are expected to persist indefinitely upon growth of an arbitrarily-thick GaN film. Even if GaN wafers are sliced from a thick HVPE-grown boule and used as a seed for additional growth, the edge dislocations are expected to persist indefinitely. Vaudo et al. [Phys. Stat. Solidi(a) 194, 494 (2002)] reported a dislocation density below 104 cm−2 within a grain of a thick HVPE film; however, the dislocation density between grains, most likely comprising predominantly edge dislocations, is expected to be much higher. In addition, strain is present in HVPE wafers due to the thermal expansion mismatch between substrate and film. This strain produces bowing upon cool down of the substrate and film after growth. The strain and bowing remains even after removal of the original substrate. Threading dislocations, strain, and bowing that are present in the substrate are expected to also be present in epitaxial layers deposited on such substrates to form light-emitting devices.
Moreover, neither absorption nor emission of light at room temperature occurs in thick HVPE GaN with a threshold at the band edge. In transmission spectroscopy, HVPE GaN absorbs with a cutoff near 370 nm, significantly shifted from the expected cutoff near 366 nm. Similarly, the photoluminescence peak at room temperature occurs at 3.35 eV, at significantly lower energy than expected. This behavior will compromise the performance of light emitting devices operating in the ultraviolet, as some of the light will be absorbed by the substrate rather than being emitted. The shifted photoluminescence peak indicates the presence of defect states that may compromise device performance.
Other widely-applied methods for growth of large area, low-dislocation-density GaN are variously referred to as epitaxial lateral overgrowth (ELO or ELOG), lateral epitaxial overgrowth (LEO), selective area growth (SAG), dislocation elimination by epitaxial growth with inverse pyramidal pits (DEEP), or the like. In some cases, such as U.S. Pat. No. 6,294,440 by Tsuda et al., a homoepitaxial light-emitting laser diode on such a substrate has been disclosed. In the case of all variations of the ELO method, heteroepitaxial GaN growth is initiated in a one- or two-dimensional array of locations on a substrate, where the locations are separated by a mask, trenches, or the like. The period or pitch of the growth locations is between 3 and 100 μm, typically between about 10 and 20 μm. The individual GaN crystallites grow and then coalesce. Epitaxial growth is then continued on top of the coalesced GaN material to produce a thick film or “ingot.” Typically, the thick GaN layer formed on the coalesced GaN material is deposited by HVPE.
The ELO process is capable of large reductions in the concentration of dislocations, particularly in the regions above the mask, typically to levels of about 105–107 cm−2. However, light emitting devices fabricated on ELO substrates typically have a surface area of at least about 104 μm2 (10−4 cm2) and still contain a substantial number of threading dislocations. In addition, an ELO GaN substrate is not a true single crystal although a number of authors do refer to ELO structures as single crystals. Each individual GaN crystallite constitutes a grain, and there is typically a low-angle grain boundary or a tilt boundary at the points where the grains coalesce. The low-angle or tilt boundaries are manifested as an array of edge dislocations and generate rather high and nonuniform stresses within the GaN. The magnitude of the crystallographic tilting depends on the details of the masking and growth conditions, but there is generally at least a low level of tilting associated with grain coalescence. Much or most of the crystallographic tilting forms directly during growth, rather than simply being a consequence of thermal expansion mismatch. The separation between the tilt boundaries is equal to the period or pitch of the original mask, or typically about 10–20 μm. The tilt boundaries will persist indefinitely through epitaxial layers grown on such a substrate. The consequence is that devices formed on such substrates will also have tilt boundaries running through them if the devices have lateral dimensions larger than about 100 μm, and particularly if they have lateral dimensions larger than about 300 μm, and even more so if they have lateral dimensions larger than about 2000 μm. The tilt boundaries in substrates or devices can be detected by a range of analytical techniques, including transmission electronic microscopy, x-ray diffraction, and x-ray topography.
The tilt-grain-boundary structure and lateral strain persists throughout an entire ingot and therefore into each substrate sliced from this ingot. In other words, no substrate sliced from such an ingot will be a true single crystal, free of tilt boundaries and lateral strain, and no large-area device fabricated on such a substrate will be free of tilt boundaries. In addition, the GaN substrate is likely to suffer from the same deficiencies in UV absorption and photoluminescence at room temperature as “standard” HVPE GaN.
Residual stress or strain in homoepitaxial GaN-based devices resulting, for example, from the presence of tilt boundaries, may accelerate the degradation of LDs or high-power LEDs. Similarly, dislocations associated with tilt boundaries may reduce the lifetime of high-power light emitting diodes and laser diodes. An example of such behavior, showing the dependence of laser diode lifetime on dislocation density, is shown in FIG. 6. Degradation of device lifetimes by dislocations may result from facilitating impurity diffusion into the active layer or from facile generation of new dislocations. Dislocations may act as non-radiative recombination centers, degrading the light emission efficiency of light emitting diodes and laser diodes. Dislocations may also increase reverse-bias current leakage, further degrading device performance. Clearly, the presence of even a single dislocation within a GaN-based light-emitting device can degrade its performance and/or lifetime.
Gallium nitride grown by all known methods contains native defects that may degrade the properties of the crystal and of devices grown thereupon. One commonly occurring native defect is the Ga vacancy which, in n-type GaN, acts as a deep, triple acceptor that compensates donors. In principle, hydrogen can bind to gallium vacancies, capping the dangling bond on 1–4 surrounding N atoms to form N—H bonds, denoted VGaH, VGaH2, VGaH3, and VGaH4. N—H bonds associated with Ga vacancies are predicted [C. Van de Walle, Phys. Rev. B 56, R10020 (1997)] to have vibration frequencies between 3100 and 3500 cm−1 and to be quite stable. However, known GaN crystal growth methods do not provide a means for passivating Ga vacancies by hydrogenation. For example, infrared transmission spectroscopy on 300–400 μm thick GaN samples grown by HVPE revealed weak absorption features near 2850 and 2915 cm−1 associated with another defect, but no absorption features between 3100 and 3500 cm−1 that could be assigned to hydrogenated Ga vacancies were observed.
U.S. Pat. Nos. 5,770,887 and 5,810,925 to Tadatomo, et al., teach the growth of double-heterostructure LEDs on GaN pseudo-substrates. These pseudo-substrates comprise GaN/ZnO multilayers rather than GaN single crystals. The ZnO served as buffer layers throughout the crystal growth process, and the process therefore required extra steps for the formation and later removal of the ZnO layers. The reference does not disclose dislocation densities that are achievable by this method. U.S. Pat. No. 6,225,650, to Tadatomo et al., discloses a multi-step ELO process to form low-dislocation-density GaN base layers. As with other ELO methods, coalescence of laterally-grown GaN will cause formation of tilt boundaries in these base layers.
Doping of GaN by rare earth metals is known to produce luminescence. For example, Lozykowski et al. [U.S. Pat. No. 6,140,669] teach the incorporation rare earth ions into GaN layers by ion implantation, MOCVD, or MBE, and annealing at 1000° C. or greater. Birkhahn et al. [U.S. Pat. No. 6,255,669] teach the fabrication of light-emitting diodes using GaN layers doped with a rare earth ion or with chromium. However, these inventions focus on thin GaN epitaxial layers rather than bulk crystals. Growth of luminescent, bulk GaN or homoepitaxial GaN that is substantially free of tilt boundaries, with a dislocation density less than 104 or 100 cm−2, is as yet unknown in the art.
Mueller-Mach et al. [WO 01/24285 A1] teach the fabrication of GaN-based light-emitting diodes on a single crystal phosphor substrate, preferably, rare-earth-doped yttrium aluminum garnet. DenBaars et al. [WO 01/37351 A1] teach the fabrication of GaN-based light-emitting diode structures on a substrate such as sapphire doped with chromium or other transition or rare earth ions. Neither set of inventors teaches the use of a transition- or rare-earth-metal-doped high quality gallium nitride substrate with fewer than 104 dislocations cm−2.
It would therefore be desirable to develop a method for forming a high quality GaN substrate, free of tilt boundaries and with a dislocation density less than 104 cm−2, on which to form homoepitaxial LED or LD devices, which would eliminate the above-mentioned problems.